Wireless communication systems are widely known in which base stations (BSs) form “cells” and communicate with terminals within range of the BSs. In LTE for example, the base stations are generally called eNodeBs or eNBs and the terminals are called user equipments or UEs. At is simplest, the arrangement is as shown in FIG. 2A with a UE 10 in communication with a single eNodeB 20. In practice, as will be understood by those skilled in the art, the eNodeB 20 is able to maintain communications with many UEs 10 simultaneously.
The direction of communication from the base station to the UE, indicated by an arrow in FIG. 2A, is referred to as the downlink (DL), and the reverse direction from the UE to the base station is the uplink (UL). Two well-known transmission modes for a wireless communication system are TDD (Time Division Duplex), in which downlink and uplink transmissions occur on the same carrier frequency and are separated in time, and FDD (Frequency Division Duplex) in which transmission occurs simultaneously on DL and UL using different carrier frequencies.
In such a system, each BS divides its available frequency and time resources in a given cell, into individual resource allocations for the user equipments which it serves. The user equipments are generally mobile and therefore may move among the cells, prompting a need for handovers of radio communication links between the base stations of adjacent cells. A user equipment may be in range of (i.e. able to detect signals from) several cells at the same time, but in the simplest case it communicates with one “serving” or “primary” cell. A wireless communication system, and the cells within it, may be operated in FDD (Frequency Division Duplex) or TDD (Time Division Duplex) mode.
Resources in the system have both a frequency dimension, divided in units of subcarriers, and a time dimension having units of a symbol time or “slot” (where a “slot” has typically a duration of seven symbol times), as indicated in FIG. 1A. The resources in the time domain are further organised in units of frames, each having a plurality of “subframes”. In one frame structure for LTE, the 10 ms frame is divided into 20 equally sized slots of 0.5 ms as illustrated in FIG. 1B. A sub-frame consists of two consecutive slots, so one radio frame contains 10 sub-frames. An FDD frame consists of 10 uplink subframes and 10 downlink subframes occurring simultaneously. In TDD, various allocations of subframes to downlink and uplink are possible, depending on the load conditions. Subframes may consequently be referred to as uplink subframes or downlink subframes.
The UEs are allocated, by a scheduling function at the eNodeB, a specific number of subcarriers for a predetermined amount of time. Such allocations typically apply to each subframe. Resources are allocated to UEs both for downlink and uplink transmission (i.e. for both downlink subframes and uplink subframes).
The transmitted signal in each slot is described by a resource grid of sub-carriers and available OFDM symbols, as shown in FIG. 1A. Each element in the resource grid is called a resource element (RE) and each resource element corresponds to one symbol. An amount of resource consisting of a set number of subcarriers and OFDM symbols is referred to as a resource block (RB) in LTE, as indicated by the bold outline in FIG. 1A.
The uplink in an LTE wireless communication system employs a variant of OFDMA called Single-Carrier FDMA (SC-FDMA). Essentially, SC-FDMA is a linearly precoded OFDMA scheme, involving an additional DFT step before OFDMA processing. Access to the uplink by multiple UEs is enabled by assigning to each UE a distinct set of non-overlapping sub-carriers. Hereby incorporated by reference is also 3GPP TS 36.300 providing an overall description of the radio interface protocol architecture used in LTE-based systems and in particular section 5.2 of 3GPP TS 36.300 relating to uplink transmission schemes.
In LTE, several channels for data and control signalling are defined at various levels of abstraction within the system.
FIG. 2B shows some of the uplink channels defined in LTE at each of a logical level, transport layer level and physical layer level, and the mappings between them. User data and also some signalling data is carried on a Physical Uplink Shared Channel (PUSCH). By means of frequency hopping on PUSCH, frequency diversity effects can be exploited and interference averaged out. The control channels include a Physical Uplink Control Channel, PUCCH, used to carry signalling from UEs including channel state information (CSI), as represented for example by channel quality indication (CQI) reports, and scheduling requests (SR). Of particular interest for present purposes, there is also a Physical Random Access Control Channel, PRACH with at the transport layer level, a corresponding Random Access Channel, RACH. In addition to the above channels, uplink resources are also allocated to reference signals, in particular a Sounding Reference Signal SRS.
Meanwhile, on the downlink (not illustrated), user data and higher layer signalling (e.g. for radio resource control (RRC)) is carried on the Physical Downlink Shared Channel (PDSCH). Other control channel signalling (e.g. for radio resource allocation) is carried by the Physical Downlink Control Channel, PDCCH. Downlink signalling includes System Information (SI) which consists of cell- and network-specific parameters needed by UEs to connect to the network. SI is divided into SI blocks (SIBs) each containing a set of related parameters.
The Physical Random Access Channel PRACH is used to carry the Random Access Channel (RACH) for accessing the network if the UE does not have any allocated uplink transmission resource. If a scheduling request (SR) is triggered at the UE, for example by arrival of data for transmission on PUSCH, when no PUSCH resources have been allocated to the UE, the SR is transmitted on a dedicated resource for this purpose. If no such resources have been allocated to the UE, the RACH procedure is initiated. The transmission of SR is effectively a request for uplink radio resource on the PUSCH for data transmission.
Thus, RACH is provided to enable UEs to transmit signals in the uplink without having any dedicated resources available, such that more than one terminal can transmit in the same PRACH resources simultaneously. The term “Random Access” (RA) is used because (except in the case of contention-free RACH, described below) the identity of the UE (or UEs) using the resources at any given time is not known in advance by the network (incidentally, in this specification the terms “system” and “network” are used interchangeably). Preambles (which when transmitted, produce a signal with a signatures which can be identified by the eNodeB) are employed by the UEs to allow the eNodeB to distinguish between different sources of transmission.
RACH can be used by the UEs in either of contention-based and contention-free modes.
In contention-based RA, UEs select any preamble at random, at the risk of “collision” at the eNodeB if two or more UEs accidentally select the same preamble. Contention-free RA avoids collision by the eNodeB informing each UE which preambles may be used.
Referring to FIG. 3, the Physical Random Access Channel PRACH typically operates as follows (for contention based access):—
(i) The UE10 receives the downlink broadcast channel for the cell of interest (serving cell).
(ii) The network, represented in FIG. 3 by eNodeB 20, indicates cell specific information including the following:
                resources available for PRACH        preambles available (up to 64)        preambles corresponding to small and large message sizes.(iii) The UE selects a PRACH preamble according to those available for contention based access and the intended message size.(iv) The UE 10 transmits the PRACH preamble (also called “Message 1”, indicated by (1) in the Figure) on the uplink of the serving cell. The network (more particularly the eNodeB of the serving cell) receives Message 1 and estimates the transmission timing of the UE.(v) The UE 10 monitors a specified downlink channel for a response from the network (in other words from the eNodeB). In response to the UE's transmission of Message 1, the UE 10 receives a Random Access Response or RAR (“Message 2” indicated by (2) in FIG. 3) from the network. This contains an UL grant for transmission on PUSCH and a Timing Advance (TA) command for the UE to adjust its transmission timing.(vi) In response to receiving Message 2 from the network, the UE 10 transmits on PUSCH (“Message 3”, shown at (3) in the Figure) using the UL grant and TA information contained in Message 2.(vii) As indicated at (4), a contention resolution message may be sent from the network (in this case from eNodeB 20) in the event that the eNodeB 20 received the same preamble simultaneously from more than one UE, and more than one of these UEs transmitted Message 3.        
If the UE does not receive any response from the eNodeB, the UE selects a new preamble and sends a new transmission in a RACH subframe after a random back-off time.
For contention-free RA, the procedure is simpler:
(i) The eNodeB configures the UE with a preamble from those available for contention-free access.
(ii) The UE transmits the preamble (Message 1) on the uplink of the serving cell.
(iii) The UE receives the RAR (Message 2) from the network, which contains an UL grant for transmission on PUSCH.
In both contention-based and contention-free RACH procedures, the RAR contains a Cell Radio Network Temporary Identifier (C-RNTI) which identifies the UE. In the contention-based procedure, the UE transmits this C-RNTI back to the eNodeB in Message 3 and, if more than one UE does so there will be a collision at the eNodeB which may then initiate the contention resolution procedure.
Situations where the RACH process is used include:                Initial access from RRC_IDLE        RRC connection re-establishment        Handover        DL data arrival in RRC_CONNECTED (when non-synchronised)        UL data arrival in RRC_CONNECTED (when non-synchronised, or no SR resources are available)        Positioning (based on Timing Advance)        
The RACH procedure can be triggered in response to a PDCCH order (e.g. for DL data arrival, or positioning). Contention free RA is only applicable for handover, DL data arrival and positioning.
FIG. 4 illustrates the format of a PRACH preamble message in LTE. As shown, the message (here denoted by 30) consists of a cyclic prefix CP 31, the preamble sequence 32 itself, and (not illustrated here) a guard interval to allow for differences in arrival timings at the eNodeB. The CP has a duration TCP 33 and the sequence 32 has a duration TSEQ 34. Note that there is no room in the conventional PRACH preamble for additional information.
Given the PRACH preamble format specified in LTE as shown in FIG. 4, the UE can only indicate limited control information with RACH Message 1. If it were possible to convey more information early in the RACH procedure, system performance could be improved. For example, delay could be reduced between initiation of the RACH procedure and start of data transmission with maximum data rate or spectral efficiency.
In LTE, the preambles transmitted by the UEs are designed to be received at the network using correlation techniques, with typically one correlator per sequence. Directly increasing the information content of a preamble by increasing the number of preambles available would impose a significant increase in complexity in the eNodeB. Therefore a different solution is needed.
Meanwhile, the advent of machine-type communications (MTC) between e.g. smart meters in homes and an LTE network creates a large number of deployed devices which must be low cost, low power, are generally deployed statically and have low-rate, possibly periodic data transmissions with potentially long gaps.
It is therefore desirable to reduce the power required to run such devices to a level significantly below that of even low-category UEs as currently specified. One way to do this is to design signalling which is more efficient than existing LTE signalling by being targeted at the MTC-device scenario. The LTE RA procedure has the following problems in such a scenario:
The contention-based RA procedure must be used in most cases. This can result in repeated need to transmit on PRACH even though in the case of MTC devices their access requirements may be tolerant to delay and have a meaningfully non-random pattern to them.
There can be significant numbers of MTC devices within the coverage of a single cell, but there are only 64 preambles available in a given cell at a given time. Certain events can trigger many of the devices to have UL data to transmit simultaneously, resulting in many RACH collisions and wasted transmissions by UEs on PRACH. It is desirable to reduce these for low-cost, low-power devices.
The RA procedure must be used whenever a UE is not uplink-synchronised, or has become unsynchronised (e.g. due to UE clock drift), a potentially frequent occurrence in transmission profiles with long silent periods and low-cost devices typical in some MTC-device scenarios.
The network has little capability to distinguish between device types early in the LTE random access procedure, so it cannot necessarily respond appropriately to each type of the mix of devices it is serving.
Little information can be exchanged between eNodeB and UE in the RA procedure, thus making poor use of the limited transmission power available in low-cost devices.